The invention relates to a drogue for in-flight refuelling.
A known in-flight refuelling drogue comprises a tapered annular parachute-like canopy supported by a plurality of rigid metal ribs. When the drogue is trailing behind an aircraft for in-flight refuelling, it will typically generate around 1,000 pounds of aerodynamic drag which increases with the square of the speed. The drag on the drogue is utilised to pull a refuelling hose from a pod on the tanker aircraft. The drag also resists docking forces from a receiver aircraft to be refuelled and it also serves to maintain stability. However, if a receiver in the form of an unmanned aerial vehicle (UAV) were subjected to the typical drag load from the drogue, the load could easily overwhelm such a small aircraft.
FIG. 1 of the accompanying drawings is a diagrammatic cross section through part of a drogue 1 and shows the way in which a plurality of rigid metal ribs 2 are pivoted at their inner ends as viewed in FIG. 1 to a fulcrum ring 3. In practice, there will be a multiplicity of such ribs equally spaced around the ring for example, forty two ribs in all. The outer ends of the ribs 2 support an annular canopy 4. In use, the drogue 1 will be stowed in a refuelling pod on the tanker aircraft and in the stowed condition, the ribs will occupy folded positions indicated in broken lines in FIG. 1. When the drogue is to be released for refuelling an aircraft, the drogue is allowed to move out of the pod and the air stream fills the canopy 4, causing the ribs 2 to be moved into the full line position in FIG. 1. The drag created pulls the drogue backwards thereby drawing the refuelling hose from the pod. The drag stabilises the drogue 1 by creating equal outward forces all around the canopy 4.
Stability of the drogue is achieved by utilising the xe2x80x9cdihedralxe2x80x9d effect of the canopy 4. The canopy 4 is angled to the airflow F such that if the angle A of the canopy 4 changes relative to the airflow F, the forces on part of the canopy 4 will increase as the cross sectional area of that part presented to the airflow F increases. On the other hand, the diametrically opposite part of the canopy 4 will present a smaller angle to the airflow F and the drag on that area deceases. The effect causes the drogue 1 to be self centring so that equal forces act all around the canopy 4. The stability created by the canopy 4 is adequate for normal aircraft being refuelled to achieve safe docking.
To achieve a docking connection when refuelling, a high degree of crew training is involved due to the way in which the drogue tends to position itself in response to airflow. It is well known that when a receiver aircraft approaches a drogue, the presence of the aircraft modifies the direction of airflow over the drogue and the drogue has no option but to follow the new airflow rather than maintain its original position relative to the tanker aircraft. As the receiver aircraft approaches the new position of the drogue the airflow over the drogue is modified again and so on. Generally, the larger the receiver aircraft the greater the airflow disturbance over the drogue. In response, the pilot of the receiver aircraft has to learn established techniques that will involve approaching the drogue quickly and aiming at a point in space to which the drogue will move in response to the approaching aircraft. To pilot the receiver aircraft correctly in such conditions involves a great deal of skill.
An object of the present invention is to provide a drogue which will be easier to position relative to a receiver aircraft.
According to a first aspect of the invention there is provided a drogue for in-flight refuelling comprising a canopy supported by a plurality of adjustable supports such as ribs, control means for moving the supports into a first position, in which the air stream can inflate the canopy whereby drag on the canopy, in use, will cause the drogue to draw a fuel hose from a tanker aircraft into an extended position, and into a second position in which drag on the canopy in the extended position will be reduced, at least one of the adjustable supports being movable relative to the others by control means to adjust a selected portion of the canopy whereby the drag on that portion will be increased and the drogue will move to a new position.
By reducing the drag, the force on an aircraft pushing forward into the drogue for refuelling will likewise be reduced, a factor which is of particular importance when it comes to in-flight refuelling of a UAV. According to a second aspect of the invention there is provided a method of controlling a drogue comprising a canopy supported by a plurality of adjustable supports such as ribs, the method comprising positioning the supports to allow an air stream to inflate the canopy such that drag on the canopy causes the drogue to draw a fuel hose from a tanker aircraft into an extended position, adjusting the adjustable supports so as to position the canopy to reduce drag on the canopy in the extended position and controlling one or more of the adjustable supports to cause drag to vary on a selected portion of the canopy and thereby cause the drogue to move to a new position.
In either the first or second aspect of the invention:
The adjustable supports preferably move the canopy into a position so that the canopy takes on a substantially cylindrical form after the hose reaches its extended position to minimise drag.
Preferably, further supports are provided for the canopy. The canopy may be arranged such that one edge is mounted on the adjustable supports and another edge is mounted on the further supports. Preferably, the further supports are pivotally connected to the adjustable supports. Some of the supports may be formed with slots in which the others of said supports are slidable. In the latter case, the further supports may be the ones formed with the slots.
The adjustable supports may be connected to mounting means which can be moved angularly to control the positions of given adjustable supports relative to others. Preferably, the mounting means can be moved axially of the drogue so as to move all adjustable supports simultaneously.
The mounting means may be movable by a control mechanism which preferably includes an angularly movable driving element. The driving element is preferably mounted for angular movement on a bearing and may take the form of a collar. Preferably, the bearing is mounted for axial movement on part of the drogue. The angularly moveable driving element may be arranged such that when the element is tipped in one direction to move selected adjustable mounting means in said one direction, another portion of the element tips in an opposite direction to move further selected adjustable mounting means in said opposite direction.
In a preferred embodiment, a controllable flying surface arrangement is proved for controlling stability of the drogue. The flying surface arrangement preferably operates in response to a signal received from at least one sensor such as an accelerometer within the drogue. Electric power for operating the flying surface arrangement can be generated using an air-driven turbine on the drogue. As the drag on the drogue will be reduced, relatively small flying surfaces can be used which, in turn, will require small actuators to drive them and will enable fast flying-surface response to be achieved.
According to a third aspect of the invention there is provided a tanker aircraft having a drogue in accordance with the first aspect of the invention or any of the subsidiary clauses relating thereto.